1. Field of the Invention
The present invention relates to a dry etching method which is employed in such applications as a production of semiconductor devices. More particularly, it relates to a dry etching method whereby an underlying layer selectivity and anisotropy may be prevented from deteriorating due to excessive radicals in an over-etching process.
2. Description of the Related Art
The recent trend toward more strict design rules for such semiconductor devices as VLSIs and ULSIs requires dry etching technologies to etch target material layers with a correspondingly higher underlying layer selectivity.
For instance, processing a MOS-FET gate electrode requires etching polysilicon layers or such silicon (Si)-based material layers as polycide films by achieving high selectivity for thin gate insulation films formed of silicon oxide (SiO.sub.2). Similarly, any attempt to form contacts in impurity-diffused regions formed in semiconductor substrates or in source and drain regions of PMOS transistors used as load resistance elements for SRAM requires etching the SiO.sub.2 interlayer insulating film by achieving high selectivity for Si-based material layers such as silicon substrates and polysilicon layers.
However, since dry etching involves a trade-off among underlying layer selectivity, anisotropy, high etchrate, low pollution, and low damage, these requirements must be balanced against one another to an industrially allowable degree.
Conventionally, CFC (chlorofluorocarbon) gases typified by CFC 113 (C.sub.2 Cl.sub.3 F.sub.3) or a mixed gas of CFC 113 and SF.sub.6 with SF.sub.6 added to the improve etchrate have been widely used to etch layers of material based on such silicons (Si) as single crystal silicon, polysilicon, refractory metal silicide, and polycide. Particularly, CFC-based gases, whose molecule contains F and Cl, allow etching using both a radical reaction and an ion-assisted reaction and sidewall protection using carbonaceous polymers deposited from the gaseous phase.
Meanwhile, CHF.sub.3 gas, mixed gas of CF.sub.4 and H.sub.2, mixed gas of C.sub.2 F.sub.6 and CHF.sub.3, and C.sub.3 F.sub.8 have been typically used to etch silicon oxide (SiO.sub.2)-based material layers. The common functions of these gases include: (a) forming a C-O bond from a constituent element C on the surface of a SiO.sub.2 layer and dissociating or weakening a Si-O bond, (b) forming CF.sub.x.sup.+ as an etchant for SiO.sub.2, and (c) forming relatively carbon-rich plasma and thereby removing oxygen from SiO.sub.2 in the form of CO or CO.sub.2 while reducing the etchrate through carbonaceous polymers deposited on Si and thereby achieving a high selectivity for Si.
However, CFC-based gases, typical of etching gases for Si-based material layers, are commonly known to contribute to destruction of the earth's ozone layer and the production and use thereof are likely to be prohibited in the near future. In these circumstances, there is pressing need to find some appropriate substitutes for such depositional carbon-based gases for use in dry etching and establish the efficient application methods thereof.
In etching processes using depositional carbon-based gases to achieve anisotropy, the future trend toward more strict design rules for semiconductor devices may permit carbonaceous polymers deposited from the gaseous phase to become particle pollutants. For instance, when Si-based material layers are etched using mixed gases of CFC 113 and SF.sub.6, the flow rate of CFC 113 is increased in an over-etching process in order to prevent anisotropy and underlying layer selectivity from deteriorating due to excessive radicals. However, such an attempt to increase the amount of depositional gases during etching raises the possibility of particle pollution. Likewise, any change in the composition of etching gas during etching delays stabilization of electric discharge conditions and consequently reduces controllability and throughput.
The present inventor has proposed a great number of methods of solving the above-mentioned problems. These methods fall roughly into two types: those which use other sidewall protection substance than carbonaceous polymers instead of using depositional carbon-based gases and those which improve the efficiency in formation of carbonaceous polymers to save the greatest possible amount of depositional carbon-based gases.
The former type of methods are intended to provide sidewall protection by sulfur (S) deposits, which will be formed in etching gas when the gas is composed mainly of sulfur halides with a relatively high S/X ratio, i.e. the ratio of the number of halogen (X) atoms to that of sulfur atoms.
Specifically, sulfur halides are sulfur fluorides such as S.sub.2 F.sub.2, sulfur chlorides such as S.sub.2 Cl.sub.2, and sulfur bromides such as S.sub.2 Br.sub.2. Serving as a main etchant for Si-based material layers, the sulfur halides can form F*, Cl*, and such ions as SF.sub.x.sup.+, SCl.sub.x.sup.+, and SBr.sub.x.sup.+ and, under some conditions, promote both radical and ion assisted reactions. Unlike SF.sub.6 well-known as an etching gas, the sulfur halides can deliver sulfur (S) when dissociated through electric discharge. Under some etching conditions, when a target substrate (wafer) is maintained at a temperature lower than about 90.degree. C., the sulfur emitted will deposit on the surface thereof, producing a sidewall protection effects. When the substrate is heated after completion of the etching, the sulfur deposits will sublime immediately, avoiding the danger of inducing particle pollution.
The present inventor has also proposed methods of promoting sulfur deposition in which H.sub.2, H.sub.2 S, silane, and other compounds capable of consuming halogen radicals are added to sulfur halides to increase the S/X ratio in an etching reaction system.
The latter type of methods are intended to etch SiO.sub.2 -based material layers by using higher fluorocarbon-based compounds which are highly efficient in forming carbonaceous polymers. Specifically, such fluorocarbon-based compounds are unsaturated linear compounds, saturated cyclic compounds, and unsaturated cyclic compounds. Each fluorocarbon-based gas forms a plurality of CF.sub.x.sup.+ ions and contributes to a higher etchrate. Further, fluorocarbon-based gas severs its carbon skeleton in ECR plasma and forms chemical species promoting polymerization, thus improving the efficiency of the formation of carbonaceous polymers.
The above-mentioned methods previously proposed are far more advantageous than the conventional ones in allowing clean etching. However, they have proved to require further refinement in order to improve underlying layer selectivity in an over-etching process.
For instance, when the above mentioned S.sub.2 F.sub.2 is used to etch Si-based material layers in gate electrode processing, highly reactive F* acts as a main etchant, making it difficult to maintain the high etchrate for gate insulation films formed of SiO.sub.2 in an over-etching process. This difficulty can be accounted for by the fact that a Si-F bond has a great bond energy of 132 kcal/mol when compared with 111 kcal/mol for an Si-O bond.
When etching SiO.sub.2 -based material layers with Si-based material layers as the underlying layer thereof, there is a pressing need to secure underlying layer selectivity because an Si-Si bond has a very small bond energy of 54 kcal/mol when compared with an Si-F bond or 96 kcal/mol for an Si-Cl bond. In other words, Si-based material layers are so vulnerable to attacks by F* or Cl* that they will be etched spontaneously even when irradiated with no ion.
The presence of excessive radicals in an over-etching process deteriorates not only underlying layer selectivity but also pattern anisotropy. Over-etching is accompanied by a sharp reduction in the area of target material layers, so that those radicals which have lost the binding mate thereof cause lateral migration on the surface of a target substrate (wafer) and attack the side wall of the pattern thereof, deteriorating anisotropy thereof. A polycide film, in particular, which consists of two laminated material layers with different etching properties, are likely to cause undercut and other shape defects because the lower polysilicon layer has a higher etchrate than the upper refractory metal silicide layer.